A conventional power transmission device is disclosed in Japanese Patent Application Publication Laid-open No. 2000-87850, and is applied to a clutchless compressor. As shown in FIG. 1 a compressor 101 includes a housing 102, a rotary shaft 104 and a power transmission device 111. The housing 102 has an end portion at which a boss portion 103 is formed. The rotary shaft 104 has an end portion 104a which passes loosely through the boss portion 103. The boss portion 103 is coaxial with the rotary shaft 104.
The power transmission device 111 includes a bearing 112, a pulley 113, a cover member 114, a hub 115, a bolt 116, a washer 117, rivets 118, buffer rubbers 119 and rolling balls 120. The pulley 113 is rotatably held by the boss portion 103 via the bearing 112. The bearing 112 and the pulley 113 are coaxial with the rotary shaft 104. The pulley 113 has an external circumference on which a belt (not shown in the figure) is wound. The belt is coupled with a crankshaft (not shown) of an engine.
The cover member 114 is formed into the shape of a disk and is fixed via the hub 115 to the end portion 104a of the rotary shaft 104 with the bolt 116 and the washer 117. Further, the cover member 114 is fastened to the hub 115 with the rivets 118. The cover member 114 and the hub 115 are coaxial with the rotary shaft 104.
As shown in FIG. 2, the cover member 114 has a periphery on which plural recessed portions 114a are formed. The recessed portions 114a are disposed along the same circumference whose center coincides with the axis of the cover member 114. Each of the recessed portions 114a is located a regular angle apart from the adjacent ones 114a. The buffer rubbers 119 are formed nearly into the shape of a column and are fastened to the interior of the recessed portions 114a with an adhesive, respectively. The buffer rubber 119 has an end face 119b, protruding from the recessed portion 114a, where the end face 119b has a concave portion 119a for receiving one part of the rolling ball 120 slidably (refer to FIG. 1). Besides, a power-transmission cutoff member is composed of the buffer rubber 119 and rolling ball 120.
The pulley 113 has hole portions 113a, at the locations opposite to the concave portions 119a, for receiving the other part of the rolling balls 120 slidably. The hole portions 113a are disposed along the same circumference whose center coincides with the axis of the pulley 113. Each of the hole portions 113a is located a regular angle apart from the adjacent ones 113a. The depth of the hole portions 113a is designed such a depth that the rolling ball 120 can be surely released from the hole portion 113a when a torque-load larger than a given value is applied to the rolling ball 120.
Openings 113b are formed along the above circumference on which the hole portions 113a are disposed, and receive the rolling balls 120 released from the hole portions 113a. The depth of the openings 113b is larger than a diameter of the rolling balls 120.
When the engine is driven, power is transmitted via the belt to the power transmission device 111 and then rotates the pulley 113. Further, the power is transmitted via the rolling balls 120, the buffer rubbers 119, the cover member 114 and the hub 115 to the rotary shaft 104.
Once burn-in occurs in the interior of the compressor 101, the rotary shaft 104 stops rotating. Following the occurrence, the hub 115 and the cover member 114 also stop rotating, and therefore the numbers of revolutions of the pulley 113 and the cover member 114 come to differ from each other, resulting in that a torque load is applied to the buffer rubbers 119. When the torque load exceeds the given value, according to the application of the torque load to the rolling balls 120 via the buffer rubbers 119, the rolling balls 120 get out of the concave portions 119a against the holding force of the buffer rubbers 119 and are simultaneously released from the hole potions 113a. And then, the rolling balls 120 will enter into the interiors of the openings 113b. Since the power transmission from the pulley 113 to the rotary shaft 104 is cut off through the above mechanism, the pulley 113 will run idle.
However, when the power transmission from the pulley 113 to the rotary shaft 104 is cut off, it is necessary to release each rolling ball 120 from the concave portion 119a of the buffer rubber 119 and the hole portion 113a of the pulley 113 which cover the whole external circumference of the rolling ball 120. Therefore, the torque load required to cut off the power transmission varies to a large extent due to wear of the concave portion 119a and/or the hole portions 113a. Further, since the torque load is applied to the rolling ball 120 via the buffer rubber 119, the torque load required to cut off the power transmission varies to a large extent due to age-degradation of the buffer rubbers 119. As a result, the power transmission device 111 possesses lower reliability because the torque load required to cut off the power transmission varies each time the device is operated. Moreover, the assembling operation is laborious and the productivity is low because the rolling ball 120 should be disposed between the pulley 113 and the buffer rubber 119.